(1) Field of the Invention
This invention relates to light emitting diode (LED) arrays, specifically high intensity LED arrays, to be used as a solid state light (SSL) source.
(2) Description of the Related Art
Currently, a white SSL source using Gallium Nitride (GaN) semiconductor power chips with a phosphor coating can generate 50-100 lumens per watt at the device level, and deliver 35-70 lumens per watt as a light source. Individual units, up to a 30-40 W range, have been produced since 2005 with a maximum light flux approaching 1400 lumens.
One approach for producing high intensity LED sources has been to group many LEDs in an array form. U.S. Pat. No. 5,793,405 to Y. Shakuda shows a linear LED chip array arranged as a print head. U.S. Pat. No. 7,055,987 to G. Staufert shows a two dimensional LED array in a luminous panel. Lamina Ceramics of Westampton, N.J. produced a flat panel of 50 LED assemblies, each powered at 28 W that reached 1400 W with a light flux output of 28,000 lumens. (LED Magazine, October 2005). This was a demo unit that never made it to production. These examples show that arrays of individual LED devices in a simple tile arrangement can be combined to create a high intensity kilo-lumen (KL) light source. However, because of the low efficiency (20-30 lumens per watt) of the individual LED sources, these panels tend to be bulky, heavy, and produce excessive heat. These elements pose serious problems at higher wattages. To understand these problems, it is necessary to explain how a LED creates white light.
The most common means to obtain white light in a LED is to mix blue and yellow light. FIG. 1 shows an emission of white light using a prior art blue LED combined with a yellow phosphor, described by U.S. Pat. No. 6,791,119 to Slater, Jr. et al. The process is as follows: a YAG (yttrium aluminum garnet) phosphor 1 is formed on the top of a blue LED 2 having an InGaN/GaN structure. In this structure, a current is injected from a lead frame 3 with a reflector cup 4 through a conductor 5 to the LED chip 2, which activates and emits blue light. The remaining current flows out to another lead frame 6. The blue light in turn excites the YAG phosphor, which emits a yellow fluorescence light. The mixture of the blue emission from the LED chip and the yellow emission from the phosphor results in a multidirectional white emission. An epoxy lens 7 improves the external emission efficiency of the white light.
So, to obtain a white light source panel having a large area, it is necessary to use a large number of LED chips. In FIG. 2, a series of three LED chips 2b, 2c, 2d are shown. In this structure, current is injected from a lead wire 6b through a connector post 5b to the LED chip 2b, which activates and emits blue light. The current flows through wire 3b and conductor post 5c to activate LED chip 2c. The current continues to flow through wire 3c and conductor post 5d to activate LED chip 2d. The remaining current continues to flow out to lead wire 3d. For the prior art design shown in FIG. 2, the driving circuit becomes complicated and the fabrication cost becomes high. Moreover, it is difficult to use a large number of prior art LEDs to provide a white light panel having uniformly distributed light emission. Further, while a thin lighting panel is preferred, as shown in FIGS. 1 and 2, the thickness and lateral dimension of the panel become large because of the thermal management requirement.
The biggest problem with a high intensity LED array is difficulty of thermal (and consequently power) control. At high wattages, LED junction temperature can limit luminosity, since light emission decreases dramatically as junction temperature increases. To reduce the junction temperature, it is necessary to provide a shorter heat conduction path between the LED junction and the metallic back plane and use high thermal conductivity materials as substrates or heat sinks. The package for an LED array light source may contain a bulky heat sink and cooling fan to keep the junction temperature in its working range, which is typically around 125° C.
Two methods of solving these intensity limiting problems exist: the matrix and flip chip approaches. The matrix approach uses many lower power LEDs to generate the needed light intensity. U.S. Pat. No. 5,893,721 to R. T. Huang et al. mentions a method of manufacturing an Active Matrix LED at a wafer level with GaAs, SiC, sapphire, and amorphous silicon. U.S. Pat. No. 6,329,676 to T. Takayama describes a method of using thin film strips of InGaN and GaN to form a mosaic of LED junctions for an effective flat panel source. An intrinsic difficulty in these approaches is that the parallel circuitry requires a high current density. So to minimize junction temperature rise, the size of the LED matrix is limited.
The best thermal management method for an LED array employs flip-chip bonding of LEDs to a ceramic sub-mount. U.S. Pat. No. 6,885,035 to Bhat et als describes a multi-chip LED semiconductor approach that bonds multiple LEDs to a sub-mount wafer using this flip-chip method. U.S. Pat. No. 6,964,877 to Chen et al. describes an LED power package using another flip-chip bonding method. U.S. Pat. No. 7,170,100 to Erchak et al. describes packaging for an LED array using various flip chip sub-mount designs. The total number of chips that can be bonded is limited to 4-6 by the practicality of applying phosphor, bonding the epoxy lens, etc. Therefore, using a 6-chip packaging as an example, the maximum luminous intensity achievable is 600 lumens with a LED device efficiency of 100 lumens per watt.
Development of an LED array which has a small footprint and yet has effective heat dissipation represents a great improvement in the field of LED manufacture and satisfies a long felt need of the lighting engineer.